The primary aims of this project are to (1) obtain a high-resolution crystal structure of the complete 70S ribosome in complex with mRNA and tRNA, and (2) solve the structures of new functional complexes of the ribosome, using X-ray crystallography. The first aim will be approached by improving parameters of purification, crystallization and data collection, and by crystallization of ribosomes from different organisms. A high-resolution structure of the complete ribosome will provide new information about the molecular basis of how the ribosome interacts with mRNA and tRNA, how the two ribosomal subunits interact with each other, and the detailed structures of molecular features that are disordered in structures of the isolated ribosomal subunits. We plan to solve the structures of new functional complexes of the 70S ribosome bound to its different functional ligands, including translational elongation factors and termination factors. We also plan to study the structures of ribosomes bound to mRNAs containing downstream pseudoknots and secondary structure elements. These studies will contribute toward the ultimate goal of understanding the mechanism of protein synthesis in terms of ribosomal dynamics at atomic resolution. This information will also be important in understanding the mechanisms of action of the many important antibiotics that target bacterial ribosomes, possibly leading to the design of novel drugs that are able to circumvent resistance mechanisms that have evolved in many pathogens. Structures of ribosomes bound to pseudoknot- and hairpin-containing mRNAs will provide a structural basis for understanding how these mRNA structural elements induce translational frameshifting events that are essential for expression of downstream genes in retroviruses and other RNA animal viruses.